A variable resistor is often used in a fuel level sensor to detect a change in fuel level in a fuel tank of an automobile. A typical variable resistor assembly has a wiper mechanically movable across contact segments of a resistor to change resistance value without interrupting a circuit to which the resistor is connected. The wiper is movable, usually in response to a float in a fuel tank responsive to changes in the level or depth of liquid fuel in the fuel tank. The typical variable resistor assembly has a resistor card including a ceramic substrate, two separate terminals on the substrate, and two separate and respective arcuate wiper contact areas on the substrate that are electrically connected with the terminals. Contact segments of at least one of the wiper contact areas are electrically connected to a resistor. Typically the wiper is pivotably mounted by an arm in relation to the substrate and bridges the wiper contact areas.
Typically, the resistance value of the variable resistor assembly varies in accordance with the position of the float. As the level of fuel within a fuel tank changes, the float member and actuator arm move and thereby cause the wiper to sweep over the arcuate wiper contact areas to change an effective length of the variable resistor between the terminals and thereby vary the effective resistance of the variable resistor. In accordance with the change in resistance, the output voltage of the resistor card changes and, thus, effects a change—such as from “Full” toward “Empty”—in a remote fuel level indicator useable by a driver in a passenger compartment of a vehicle.
In use, existing fuel level sensors can fail as a result of wiper and contactor wear associated with hundreds of thousands of resistor cycles and as a result of conductive contact segments reacting with liquid fuel or byproducts or additives contained within the liquid fuel. In an effort to combat such failure of fuel level sensors, various manufacturers have designed conductive wiper contactors and conductive contact segments composed of materials having an increased durability in the presence of a hostile fuel tank environment. Included in these materials are precious metals such as platinum, gold, silver, and palladium, which can be combined into alloys. Unfortunately, however, the cost of using such expensive alloy materials greatly contributes to the overall cost of a fuel level sensor assembly and such alloyed conductive layers are usually relatively unstable and require one or more plating and/or coating steps. The alloyed conductive contact segments require one or more layers of plating of nickel or nickel alloy material, and resistive portions of the resistor must be coated with an insulative protective coating prior to plating the alloyed conductive layers to prevent the resistive portions from becoming plated.
Moreover, existing fuel level sensors do not provide a means for preventing corrosion-inducing leakage currents between high potential areas. The electrical potential on a variable resistor card is highest between the terminals, is relatively high between respective electrically conductive pathways leading away from the terminals, and gradually decreases to near zero as the wiper and hence the circuit proceeds to distal portions of the wiper contact areas. Where the respective electrically conductive pathways are relatively thin and/or where the distance between the respective electrically conductive pathways is relatively small, electrical current has a tendency to leak therebetween. Such current leakage leads to corrosion of the electrically conductive pathways, the buildup of deposits, and eventual failure of the variable resistor assembly.